The present invention relates to an RF variable gain amplifying device having a switch element for switching gain between different levels.
In a radio communication system represented by a mobile phone, a low noise characteristic and a high gain characteristic are required of an amplifying circuit in the initial stage of a receiving device when a small signal is received. In addition, a low distortion characteristic and a low gain characteristic are also required when a large signal is received. In recent mobile communication, in particular, the intensity of an electric field during reception greatly changes depending on the distance between a base station and a mobile station so that the receiving device requires a wide dynamic range. As a result, the low-noise amplifying circuit of a receiving front end unit is required to have a gain control function.
As an example of such an amplifying circuit having the gain control function, an RF variable gain amplifying device is disclosed in Japanese Laid-Open Patent Publication No. 10-173453.
FIG. 9 shows the RF variable gain amplifying device disclosed in the publication, which is an amplifying circuit in one of plural stages of amplifying circuits.
As shown in FIG. 9, an RF variable gain amplifying circuit 300 comprises: an amplifying circuit 310: and an RF switch element 320Q composed of a field effect transistor (FET) for opening and closing the signal bypass circuit of the amplifying circuit 310.
Between the amplifying circuit 310 and input and output nodes Ti and To, respective dc blocking capacitors Ci and Co are connected in series.
The amplifying circuit 310 includes an amplifying element 311 composed of a FET in a source-grounded configuration. To the gate of the amplifying element 311, an RF signal is supplied from an input node Ti via an input matching circuit 312 and a specified gate bias voltage Vgg is supplied through a resistor 313. The drain of the amplifying element 311 outputs an amplified signal to an output node To via an output matching circuit 315, while it is connected to a selection switch circuit 305 via a drain bias supply circuit 316.
The selection switch circuit 305 has a first input node a connected to a supply line for a power supply voltage Vdd and a second input node g which is grounded. Switching between the nodes a and g is performed by a power supply control circuit 307 which operates in accordance with power transmission control data or on the level of a received signal.
The RF switch element 320Q uses a depletion-type FET having a drain connected to the input node Ti via a dc blocking capacitor 303c, a source connected to the drain of the amplifying element 311, and a gate connected to the ground via the resistor 306.
In the conventional RF variable gain amplifying device 300 thus constructed, the power supply voltage Vdd is supplied to the amplifying circuit 310 via the selection switch circuit 305 under the control of the power supply control circuit 307, while the selection switch circuit 305 is connected to the first input node a, whereby the amplifying circuit 310 is brought into an operating state. The power supply voltage Vdd applied via the selection switch circuit 305 is supplied as a control signal to the RF switch element 320Q so that a gate-source voltage in the RF switch element 320Q is lowered to bring the RF switch element 320Q into an OFF state and bring the connection of the signal bypass circuit into an open state. In the open state, a high-gain operating mode is established in which the level of an output RF signal at the output node To is higher than the level of an input RF signal at the input node Ti by the gain of the amplifying circuit 310.
If the selection switch circuit 305 is switched to the second input node g, on the other hand, the supply of the power supply voltage Vdd to the amplifying circuit 310 is cut off so that the operation of the amplifying circuit 310 halts, while a ground potential is supplied as a control signal via the selection switch circuit 305 to bring the RF switch element 320Q into an ON state and bring the signal bypass circuit into a connected state. When the signal bypass circuit is in the connected state, a low-gain operating mode is established in which the level of the output RF signal at the output node To is lower than the level of the input RF signal at the input node Ti by the sum of a loss resulting from the insertion of the RF switch element 320Q and a loss resulting from mismatched impedances at the input node Ti.
By thus switching the RF amplifying device to the high-gain operating mode if the level of the RF signal inputted to the input node Ti is lower than a specified level and switching the RF amplifying device to the low-gain operating mode if the level of the RF signal inputted to the input node Ti is higher than the specified level, the RF amplifying device capable of handling an RF signal with a wide dynamic range can be implemented.
When the conventional RF variable gain amplifying device is in the high-gain operating mode in which the selection switch circuit 305 is switched to the first input node a, however, the drain of the RF switch element 320Q is brought into a floating state. This causes the problem that a residual charge in the floating state varies a drain potential and the varied drain potential varies isolation when the RF switch element 320Q is in the OFF state.
It is therefore an object of the present invention to prevent varying isolation when an RF switch element provided in a signal bypass circuit is in the OFF state by solving the foregoing conventional problems.
To attain the object, the present invention constructs an RF variable gain amplifying device such that the electrode of the switch element for signal bypass circuit is not brought into the floating state.
Specifically, a first RF variable gain amplifying device according to the present invention comprises: an amplifying circuit; a switch element connected in parallel with the amplifying circuit; and a resistor connected in parallel with the amplifying circuit and with the switch element, the amplifying circuit not operating when the switch element is in an ON state but operating when the switch element is in an OFF state, a potential at each of input and output terminals of the switch element being lower when the switch element is in the ON state than when the switch element is in the OFF state.
Since the first RF variable gain amplifying device comprises the resistor connected in parallel with the amplifying circuit and with the switch element, the electrode of the switch element is not brought into the floating state even if the switch element is in the OFF state and the amplifying circuit is in the ON state, i.e., in a high gain state (in the high-gain operating mode). Accordingly, isolation does not vary and isolation provided by the switch element is increased. If the switch element is in the ON state and the amplifying circuit is in the OFF state, i.e., in a low gain state (in the low-gain operating mode), on the other hand, an insertion loss when the switch element is in the ON state is reduced.
In the first RF variable gain amplifying device, the amplifying circuit preferably has a bipolar transistor, an input signal to the amplifying circuit is preferably inputted to a base of the bipolar transistor, and an output signal from the amplifying circuit is preferably outputted from a collector of the bipolar transistor. In the arrangement, if a base potential is adjusted to approximately zero so that a collector current becomes approximately zero when the amplifying circuit is in the OFF state, a potential at each of the input and output terminals of the switch element can be lowered more reliably when the amplifying circuit is in the OFF state (when the switch element is in the ON state) than when the amplifying circuit is in the ON state (when the switch element is in the OFF state).
In the first variable gain amplifying device, the amplifying circuit preferably has a first bipolar transistor and a second bipolar transistor having an emitter connected to a collector of the first bipolar transistor, an input signal to the amplifying circuit is preferably inputted to a base of the first bipolar transistor, and an output signal from the amplifying circuit is preferably outputted from a collector of the second bipolar transistor. In the arrangement, the first and second bipolar transistors are in a cascode connection. This allows amplification of an input signal having a higher frequency. If a potential at the base of the second bipolar transistor is adjusted to approximately zero, the second bipolar transistor does not operate even if a signal of a strong electric field is inputted to the first bipolar transistor.
In this case, a potential at a base of the second bipolar transistor is preferably lower when the switch element is in the ON state than when the switch element is in the OFF state. In the arrangement, a potential at the collector of the first bipolar transistor becomes lower than the power supply voltage. This allows a reduction in the level of distortion of the output signal in the low-gain operating mode.
In the first RF variable gain amplifying device, a strip line is preferably provided between the input terminal of the amplifying circuit and the switch element. By properly setting the characteristic impedance or length of the strip line, the arrangement allows the input impedance of the amplifying circuit to be changed so that input and output impedances in the high-gain operating mode are brought closer to those in the low-gain operating mode. This reduces a loss resulting from mismatched impedances at the input and output terminals in each of the high-gain and low-gain operating modes.
In the first RF variable gain amplifying device, a strip line is preferably provided between the output terminal of the amplifying circuit and the switch element.
In the first RF variable gain amplifying device, a coplanar line is preferably provided between the input terminal of the amplifying circuit and the switch element.
In the first RF variable gain amplifying device, a coplanar line is preferably provided between the output terminal of the amplifying circuit and the switch element.
In the first RF variable gain amplifying device, an inductor element is preferably provided between the input terminal of the amplifying circuit and the switch element.
In the first RF variable gain amplifying device, an inductor element is preferably provided between the output terminal of the amplifying circuit and the switch element.
In the first RF variable gain amplifying device, another resistor is preferably provided in a stage subsequent to the switch element. The arrangement reduces variations in loss resulting from the insertion of the switch element and thereby widens the dynamic range of an inputted RF signal.
In the first RF variable gain amplifying device, the switch element is preferably composed of a field effect transistor having a gate electrode formed on a semiconductor substrate and source and drain layers of a first conductivity type each formed in the semiconductor substrate, the source and drain layers are preferably formed in a first well of a second conductivity type of the semiconductor substrate, and the first well is preferably formed in a second well of the first conductivity type of the semiconductor substrate. In the arrangement, the application of respective reverse bias voltages between the first and second wells and between the second well and the semiconductor substrate forms a depletion layer due to a pn junction, which dielectrically isolates each of the wells from the semiconductor substrate. As a result, a loss in input signal resulting from the leakage of an inputted RF signal to the semiconductor substrate can be reduced when the switch element is in the ON state.
In this case, respective resistors are preferably provided between the gate electrode and a voltage control node, between the first well and a ground node, and between the second well and a power supply voltage node. The arrangement reduces a loss in input signal resulting from the leakage of the inputted RF signal from the gate electrode, the first well, and the second well to a ground for RF.
Alternatively, respective inductor elements are preferably provided between the gate electrode and a voltage control node, between the first well and a ground node, and between the second well and a power supply voltage node.
A second RF variable gain amplifying device according to the present invention comprises: first and second amplifying circuits for receiving first and second input signals, respectively; first and second switch elements connected in parallel with the first and second amplifying circuits, respectively; a first resistor connected in parallel with the first amplifying circuit and with the first switch element; and a second resistor connected in parallel with the second amplifying circuit and with the second switch element, the first amplifying circuit not operating when the first switch element is in an ON state but operating when the first switch element is in an OFF state, the second amplifying circuit not operating when the second switch element is in an ON state but operating when the second switch element is in an OFF state, a potential at each of input and output terminals of the first switch element being lower when the first switch element is in the ON state than when the first switch element is in the OFF state, a potential at each of input and output terminals of the second switch element being lower when the second switch element is in the ON state than when the second switch element is in the OFF state.
Since the second RF variable gain amplifying device according to the present invention adopts a configuration of a differential amplifying circuit, isolation when the first and second switch elements are in the OFF state is increased in the high-gain operating mode in which the first and second amplifying circuits are in the ON state and the first and second switch elements are in the OFF state. In the low-gain operating modes in which the first and second amplifying circuits are in the OFF state and the first and second switch elements are in the ON state, an insertion loss when the first and second switch elements are in the ON state is reduced.
In the second RF variable gain amplifying device, a third resistor is preferably provided between the output terminal of the first amplifying circuit and the first switch element, a fourth resistor is preferably provided between the output terminal of the second amplifying circuit and the second switch element, a fifth resistor is preferably provided between respective input terminals of the third and fourth resistors, and a sixth resistor is preferably provided between respective output terminals of the third and fourth resistors. In the arrangement, the third, fourth, fifth, and sixth resistors constitute a xcfx80-type attenuator. This allows the attenuation of an RF signal inputted in the low-gain operating mode. In addition, variations in low-gain operation are reduced. Since the xcfx80-type attenuator allows the adjustment of input and output impedances, the input and output impedances in the high-gain operating mode can be brought closer to those in the low-gain operating mode. This reduces a loss resulting from mismatched impedances at the input and output terminals in each of the operating modes.
In the second RF variable gain amplifying device, the first and second amplifying circuits preferably have respective bipolar transistors, an input signal to the first amplifying circuit is preferably inputted to a base of the bipolar transistor of the first amplifying circuit, while an output signal from the first amplifying circuit is outputted from a collector of the bipolar transistor of the first amplifying circuit, and an input signal to the second amplifying circuit is preferably inputted to a base of the bipolar transistor of the second amplifying circuit, while an output signal from the second amplifying circuit is outputted from a collector of the bipolar transistor of the second amplifying circuit. In the arrangement, a potential at each of the input and output terminals of the first and second switch elements can be reduced more reliably when the first and second amplifying circuits are in the OFF state (when the first and second switch elements are in the ON state) than when the first and second amplifying circuits are in the ON state (when the first and second switch elements are in the OFF state).
In the second RF variable gain amplifying device, the first amplifying circuit preferably has a first bipolar transistor and a second bipolar transistor having an emitter connected to a collector of the first bipolar transistor, an input signal to the first amplifying circuit is preferably inputted to a base of the first bipolar transistor, while an output signal from the first amplifying circuit is outputted from a collector of the second bipolar transistor, the second amplifying circuit preferably has a third bipolar transistor and a fourth bipolar transistor having an emitter connected to a collector of the third bipolar transistor, and an input signal to the second amplifying circuit is preferably inputted to a base of the third bipolar transistor and an output signal from the second amplifying circuit is preferably outputted from a collector of the fourth bipolar transistor.
In this case, a potential at a base of the second bipolar transistor is preferably lower when the first switch element is in the ON state than when the first switch element is in the OFF state and a potential at a base of the fourth bipolar transistor is preferably lower when the second switch element is in the ON state than when the second switch element is in the OFF state.
In the second RF variable gain amplifying device, respective strip lines are preferably provided between the input terminal of the first amplifying circuit and the first switch element and between the input terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, respective strip lines are preferably provided between the output terminal of the first amplifying circuit and the first switch element and between the output terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, respective coplanar lines are preferably provided between the input terminal of the first amplifying circuit and the first switch element and between the input terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, respective coplanar lines are preferably provided between the output terminal of the first amplifying circuit and the first switch element and between the output terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, respective inductor elements are preferably provided between the input terminal of the first amplifying circuit and the first switch element and between the input terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, respective inductor elements are preferably provided between the output terminal of the first amplifying circuit and the first switch element and between the output terminal of the second amplifying circuit and the second switch element.
In the second RF variable gain amplifying device, each of the first and second switch elements is preferably composed of a field effect transistor having a gate electrode formed in a semiconductor substrate and source and drain layers of a first conductivity type each formed in the semiconductor substrate, the source and drain layers are preferably formed in a first well of a second conductivity type of the semiconductor substrate, and the first well is preferably formed in a second well of the first conductivity type of the semiconductor substrate.
In the second RF variable gain amplifying device, respective resistors are preferably provided between the gate electrode and a voltage control node, between the first well and a ground node, and between the second well and a power supply voltage node.
In the second RF variable gain amplifying device, respective inductor elements are preferably provided between the gate electrode and a voltage control node, between the first well and a ground node, and between the second well and a power supply voltage node.